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This is an easy to build broadcast quality AM transmitter that with a few component changes can operate from Medium Wave 500 KHz up to 30 MHz.
The output transistor can be a bd139. However this is only true, if we keep the frequency below 8 megahertz. So this is fine for medium wave which has a center frequency around one megahertz. And it is also suitable for ham radio in the 40 m range.
The power amplifier needs work. This won't fly! Brought the supply voltage down to 12V, which is more realistic.
Check http://www.voacap.com – where you can see that this transmitter, at about 18:00 in summer, will reach most of Europe assuming an arbitrary Transmitting Location of Paris, France.
The current design is set up to use 1 MHz, but this is not a standard medium wave frequency in any country, and you will find a good antenna for medium wave is quite hard to build. (enormous)
6925 kHz is a good short-wave frequency for Pirate operation. However, if you want to transmit legally, this could be brought up to somewhere in the 40 meters HAM band. Of course, you then cannot transmit radio program style content. You are limited to speech only, and even then, you must only talk about matters of a pecuniary nature!
It terms of power, the 5 watts that this transmitter pushes out may seem like a drop in the ocean when compared to the BBC MW AM transmitter at 500 K watts. However, this is not a good way of looking at the situation. Indeed, since the power to coverage ratio is logarithmic, in reality 5 watts is only 10 times lower in power to 500 k watts!
WARNING!
I don't advocate breaking the law or regulations, but I do recommend experimentation; building a pirate transmitter is fun and educational! This circuit can be built and experimented with legally if you take proper precautions, such as connecting a dummy load to the antenna output, which will limit the range of your signal to a short distance, likely within your home or property.
If you hook up a real antenna to the transmitter (yes, any length of wire is an antenna) you risk unlicensed broadcast
In some countries, radio piracy is unfortunately considered a criminal act. They don't want uncontrolled opinion on the air.
So, you have been warned! The bottom line is… experiment safely and cautiously, regarding others using the radio spectrum.
If you are interested in experimenting with transmitters, I encourage you to get a ham radio license. You won't be allowed to broadcast a radio show, but a ticket does allow you to legally play with transmitters on a variety of interesting radio bands. What could be better than that?!
* This TX is designed to make use of a DIL package Xtal oscillator – but this could be any 5V signal source.
* You could excite this with the frequency agile Si5351A I2C 25MHZ Clock Generator 8KHz to 160MHz – These are now available on breakout boards at very low cost from flea-bay
QUALITY
FM is undoubtedly higher quality and has the benefit of stereo from broadcast transmitter stations. However, FM is operated at UHF and VHF, which has its limitations. At these frequencies, the signal will be significantly attenuated by walls, hills and other structures. Whereas, medium wave and the higher frequency short-wave do not suffer this phenomenon.
CIRCUIT DESCRIPTION
There is a Long tail pair arrangement, with the current modulated by the audio at the current source transistor. The RF is injected as a square wave, which is OK as this pulse is subsequently filtered. The advantage of using a long tail pair to modulate is that the result is very controlled. Furthermore, the level of audio needed is 1V RMS (line level). This works well with MP3 players and other audio devices. No massive transformer is needed nor huge audio amplifier.
This example produces a QRP AM signal at 5 watts RMS into a 50 OHM load.
The output final makes use of a hack which is copied from the grenade and other similar designs. This uses a 100 ohm resistor in parallel with the load choke of the final transistor. This effectively calibrates the output impedance to be nearer to 50 ohms… in theory. It is also a Q dampener, which works well for AM.
The circuit should represent an advance on the grenade because of the above circuitry.
The long tail pair mixer already requires a transformer, so the hack to get lower Q and better 'calibration' of the output impedance avoids yet another transformer.
In this example, the circuit operates at 25 volts, but you could experiment.
I ASK FOR CONSTRUCTIVE CRITICISM
Enjoy
73
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